slit fricative [t̞]

On a certain social networking site to which I belong, Tim Morley asked

Can someone explain to me, in terms of tongue position or whatever's relevant, the scouse allophone of /t/ that's a sibilant rather than a stop?
A French colleague asked me why a scouser had referred to "The Cass in the Hass", and although I believe I can perceive a difference in the cat-cass minimal pair through the scouse accent (i.e. I don't think it's just context that's allowing me to correctly interpret a homophone), I couldn't actually explain the difference that I believe I'm hearing, which kind of annoyed me to be honest!

So I said

It’s an alveolar slit fricative rather than plosive — a kind of lenition. It can be lenited in some words even further to [h]. See my blog for 15 Nov 2006.

Tim followed up with

By "slit", do you mean simply that there's "only a slit" (i.e. a very narrow gap) between tongue and alveolus? Or is there some fuller meaning that I'm not aware of?

So I clarified.

It means the space between the tongue tip/blade and the alveolar ridge is a left-to-right gap ("slit") rather than the front-to-back "groove" you get in [s]. [θ] and [ð] are dental slit fricatives, [s] and [z] are alveolar groove fricatives. The Scouse thing combines the shape of the first pair with the place of the second.

Then Tim discovered a Wikipedia claim that "no language is known to contrast a grooved and non-grooved sibilant".

Does this mean that Scouse would constitute a counterexample to this supposed universal and thereby disprove it? Hardly. I pointed out that

the nongrooved sibilant is not the default realization nor the most frequent realization of Scouse /t/. […The opposition exemplified in hit/hiss' operates] only in final or prevocalic position, and it’s not the only possibility even there. Note that in hit back (vs hiss back) you probably wouldn't get the slit fricative, but a no-audible-release [ʔ], [p] or [t].

Kevin Watson adds:

In a word like quite or internet you'd get a slit-t (of varying kinds, I've called them e.g. 'dynamic sibilants' and 'canonical sibilants' although I'm not happy with those terms) but in what or biscuit you'd get [h].

This same [t̞] articulation of /t/ in postvocalic position is found in Irish English (“soft t”). Indeed, it’s one of the most indexical features of a (southern) Irish accent, and in Liverpool obviously derives from Irish influence.